245/65R21 vs 255/65R21
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/65R21 | 255/65R21 | Change |
|---|---|---|---|
| Overall height | 33.5" | 34.1" | +1.5 % |
| Sidewall | 6.27" | 6.53" | +4.1 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 105.4" | 107.0" | +1.5 % |
| Revs per mile | 601 | 592 | -1.5 % |
| Axle height | 16.77" | 17.03" | +6.5 mm |
| Approved rim width | 7–9" | 7–9.5" | overlap 7–9" |
Both are approved on a rim 7" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/65R21. Fit 255/65R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.4 mph | +1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,153 on the road, so it under-counts by 153. At a true 70 mph the first turns 702 times a minute, the second 691.